Water Resources in Arid Regions 
J. TIXERONT 
Chief Engineer of Public Works, Tunis 
In general we consider as arid those regions where water is 
the usual limiting factor of agricultural production. In North 
Africa, these regions are approximately those that have an 
annual rainfall of less than 500 m/m (6). They present extremely 
varied characteristics. A Tunisian arid area such as the Sahel 
has a population density of more than 70 inhabitants per square 
kilometer, although the annual rainfall is well below 400 milli- 
meters. In the south, on the other hand, where the rainfall is 
less than 100 millimeters, the population density is insignificant, 
except for the oases. 
Arid regions often draw their water resources from humid 
regions on the outside. This is so, for example, in the Nile Valley, 
in Mesopotamia, and in the valley of the Indus. This study is 
limited to the cases of typically arid areas and to the water re- 
sources contained within their perimeters. 
The hydrology of arid regions naturally obeys the same physical 
laws as the hydrology of every other region, but also has particular 
traits to which we shall limit ourselves. We realize that our experi- 
ence is related especially to the arid areas of North Africa and, 
to a lesser degree, to the rest of the Mediterranean basin. Ac- 
cordingly, to that extent this study lacks generality. There will 
be no discussion of the cold arid zones. 
In the study of water resources, it will be impossible to con- 
sider individually the agricultural and economic phases since 
these are inseparable from the hydrology in arid regions. 
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